In extracorporeal blood treatment (e.g. hemodialysis) dialysis fluid flows within a dialyzer around blood of the patient. For determining, for example, the pressure, the temperature, the flow rate of the dialysis fluid, a possible blood leakage, the conductivity, the absorbance and, respectively, extinction or fluorescence of substances usually eliminated with the urine and/or electrolytes and/or the pH value, dialysis machines frequently include plural individual sensors at the outlet for used dialysis fluid of the dialyzer.
Some of said sensors merely constitute safety measures. For example, a sensor serving as blood leakage detector in the event of membrane rupture detects blood flows into the dialysis fluid which have to be detected in due time so as to avoid high blood loss of the patient. Other sensors in turn establish rather uncritical parameters such as the dialysis dose Kt/V. For establishing the dialysis dose Kt/V optical sensors are employed, for example, which measure the absorption of light in the dialysis fluid from which the presence of substances usually to be eliminated with the urine can be concluded. Other methods are based on simultaneous conductivity measurement on the dialysis side upstream and downstream of the dialyzer.